1. Field of the Invention
The present invention relates to an alkaline secondary battery such as a nickel-hydrogen secondary battery and a nickel-cadmium secondary battery and the like, and to a nickel electrode for alkaline secondary battery used for a positive electrode of the alkaline secondary battery as described above, and in particular, is characterized in that charge/discharge cycle performance of the alkaline secondary battery under high temperature conditions is improved upon bettering the nickel electrode for alkaline secondary battery obtained by applying a paste containing active material particles comprising nickel hydroxide to a conductive substrate and then drying the paste on the conductive substrate.
2. Description of the Related Art
A conventional alkaline secondary battery such as a nickel-hydrogen secondary battery and nickel-cadmium secondary battery, has used as a positive electrode a nickel electrode for alkaline secondary battery using nickel hydroxide as the active material.
In the above-mentioned nickel electrode for alkaline secondary battery, conductivity of nickel hydroxide used as the active material is low. Accordingly, a sintered nickel electrode has been conventionally used obtained by impregnating a sintered substrate prepared by filling a conductive substrate such as a punched steel plate and the like with nickel powder and then sintering the conductive substrate having the nickel powder filled therein, with nickel hydroxide as the active material.
However, in the case of the sintered nickel electrode as described above, a problem exists. Bond between powder in nickel powder is weak, whereby nickel powder drops easily from the conductive substrate when a porosity in the sintered substrate is high. Therefore, a porosity in the sintered substrate can not be higher than around 80% on practical use, and thereby it is impossible to fill sufficiently nickel hydroxide as the active material, making it difficult to obtain an alkaline secondary battery having large capacity.
Further, another problem exists. In the case of the above-mentioned sintered nickel electrode, a filling density of the active material is generally small due to the use of the conductive substrate such as the punched steel plate. Additionally, pores of nickel powder formed by sintering are as small as not more than 10 μm. Therefore, in filling the active material, a solution impregnating method of repeating cycles of laborious process needs to be adapted, whereby productivity of filling is degraded.
As a result, a paste-type nickel electrode for alkaline secondary battery has become to be used obtained by applying a paste prepared by adding an aqueous solution of a binding agent such as methyl cellulose and the like to the active material particles comprising nickel hydroxide and then kneading a resultant mixture, to a conductive substrate having a large porosity such as nickel substrate foam and the like, and then, drying the paste on the conductive substrate.
In the case of the paste-type nickel electrode for alkaline secondary battery, the conductive substrate having a porosity of not less than 95% may be used. Therefore, it is possible to obtain an alkaline secondary battery having large capacity upon filling the sintered substrate with a large amount of active material and to facilitate filling the conductive substrate with active material. As a result, productivity of filling is improved.
However, in the paste-type nickel electrode for alkaline secondary battery, the conductive substrate is filled with a large amount of active material. Accordingly, when the conductive substrate having a large porosity is used, there exists a problem that collectivity of the conductive substrate is degraded, and thereby, rate of utilization of the active material is also degraded.
Therefore, in recent years, in the paste-type nickel electrode for alkaline secondary battery, conductivity inside an electrode has been improved, to increase the rate of utilization of the active material upon adding cobalt metal and a cobalt compound comprising cobalt oxide and cobalt hydroxide which are conductive agents to the active material particles comprising nickel hydroxide and then oxidizing by charging the above-mentioned cobalt metal and cobalt compound into cobalt oxyhydroxide (β-CoOOH).
However, even in the case where cobalt metal and a cobalt compound which are conductive agents are added to the active material particles comprising nickel hydroxide, when charge is performed to an alkaline secondary battery using the paste-type electrode for alkaline secondary battery as a positive electrode of an alkaline secondary battery, oxygen evolution overvoltage of the positive electrode is low. Accordingly, there exists a problem that a side reaction of oxygen evolution reaction occurs in addition to charge reaction of nickel hydroxide oxidized into nickel oxyhydorxide, whereby charge performance is degraded.
In this connection, in the paste-type nickel electrode for alkaline secondary battery, proposals have been made in the art as follows. For instance, Japanese Laid-Open No. Hei 8-222213 discloses a nickel electrode for alkaline secondary battery aimed at the improvement in charge performance under high temperature conditions upon increasing oxygen evolution overvoltage of the positive electrode by adding conductive agents comprising cobalt metal and a cobalt compound to a surface of active material particles comprising nickel hydroxide as well as by adding a tungsten compound and the like to the active material particles.
However, in the case of using the paste-type nickel electrode for alkaline secondary battery obtained by adding a tungsten compound and the like in addition to the conductive agents comprising cobalt metal and a cobalt compound to the surface of the active material particles comprising nickel hydroxide, when charge/discharge is performed under high temperature conditions, discharge depth at discharge is large. Accordingly, cobalt oxyhydroxide into which cobalt metal and a cobalt compound are oxidized as described above, is reduced into cobalt hydroxide. Said cobalt hydroxide is included as solid solution into an alkaline electrolyte solution of the alkaline secondary battery, after which, is precipitated on the surface of the active material particles.
In the case where cobalt hydroxide is included as solid solution into the alkaline electrolyte solution and is precipitated as described above, speed of the above-mentioned process of dissolving and precipitating is high. Therefore, when the charge/discharge is repeatedly performed under high temperature conditions, cobalt hydroxide is not precipitated uniformly on the surface of the active material particles comprising nickel hydroxide. Accordingly, cobalt hydroxide is unevenly precipitated on the active material particles and part of cobalt hydroxide is diffused into pores of the active material particles. Consequently, there exists a problem that conductivity of the nickel electrode of alkaline secondary battery is gradually decreased, and thereby, charge/discharge cycle performance under high temperature conditions is also degraded.